1. Field of the Invention
The present invention relates to digital data transmission systems using coherent light wave modulation and the construction of such systems using modulation of the frequency shift type. Use thereof is more particularly envisaged for ground-air or air-air links.
Generally, such a system uses a laser generator for producing a first coherent light wave on emission; this wave is frequency modulated on each side of a mean value F.sub.E by a modulator receiving a modulation signal translating the information to be transmitted. On the reception side, the system uses a photo-mixer for mixing the wave received with a local coherent light wave so as to provide an electric beat signal at an intermediate frequency. Processing circuits then demodulate the beat signal.
2. Description of the Prior Art
Digital data transmission techniques are also known using radio electric waves. The message may be translated by two symbols; this is the case of frequency shift modulation of the FSK type (frequency shift keying). The message may also be translated on an N basis by a succession of symbols chosen from an alphabet with N elements designated by the figures from 1 to N, N being greater than 2. This is the case of frequency shift modulation of the MFSK type (multiple frequency shift keying). Each state to be transmitted is characterized by a frequency; the figure 1 corresponds to a frequency F.sub.1, the figure 2 to a different frequency F.sub.2 and so on up to figure N corresponding to a frequency F.sub.N.
If coding of the data is of the binary type, only two symbols are available. This is the case with the FSK system where P successive data are required for translating the number 2.sup.P -1, for example 6 figures for translating the number 63 which is equivalent to 2.sup.6 -1 which is written 111111 in binary code. In the case of data coding with N symbols available, N being greater than 2, Q successive data are required for translating the N.sup.Q -1, for example on an 8 basis with N=8 different symbols only two data are required for writing the number 63. With such a code a greater amount of data may thus be transmitted than with a binary code for the same number of figures.
Data transmission systems using modulation of the type with frequency shifting of a non luminous radio electric wave do not allow a greater number of frequencies to be used, the frequency of the carrier wave being of the order of 100 MHz. The frequency band which may be used with a light wave of smaller wave length allows a greater number of frequencies to be used, the frequency of the carrier wave being of the order of 10.sup.7 MHz. Now these waves are much rarer than those whose frequencies are of the order of 100 MHz, and the risks of jamming are therefore greatly reduced. Advantage may be taken of the high directivity of the laser wave beam.
The aim of the invention is to provide a high flow digital data transmission system, using the optical path as transmission medium and a modulation signal of the MFSK type for translating the information.